Why trees attract rain

It seems that the role played by trees and forests in general is an important role and, in part, the presence of forests is directly related to rainfall. There are various mechanisms and processes involved in this relationship, many of which are still unknown.

As a curious fact, the relationship between rain and trees was already recognized for quite some time. In the biography published on Christopher Columbus by his son, the admiral himself related the sudden increase in wind speed and storms in the areas near Jamaica with the great vegetation cover, since the climate, sky and air were the same as those of other areas of the world. He himself had already observed that this phenomenon occurred in the Madeira, Canary and Azores Islands, where there was also a large vegetation cover.

This theory suggests that the high amounts of rainfall in some continental interiors, such as the Amazon in South America and the Congo in Africa, is made possible by the large forest cover that extends from the coast to the interior. It is the atmospheric physical processes, condensation and evaporation , which carry humid air towards the interior of the continents. According to this theory, deforestation can also change from a continent with high rainfall to one with low rainfall.

The biotic pump theory was proposed by Anastassia Makarieva and Victor Gorshkov and it proposes that large forest covers change atmospheric gradients and, therefore, give rise to conditions of strong winds that move moisture from the ocean towards the continental interior to replace the water displaced from the rivers into the sea. On the other hand, large forest masses evaporate a large amount of moisture , even greater than that of the ocean. As a consequence, this high atmospheric humidity leads to intense condensation, which in turn generates more air flow from the sea towards the interior. Finally, these processes give rise to large tropical storms in the oceans and high rainfall in the interior of the Amazon.

The relationship between trees and cloud formation is complex and there is currently research on the subject.

Clouds are produced when water vapor condenses or freezes from a gaseous state to a liquid or solid. This condensation takes place when the air is saturated with water. The saturation of an air mass depends on its temperature and the existence of condensation nuclei. Air saturated with water and with many condensation nuclei becomes saturated at lower humidity levels.

A large part of the atmospheric particles that are detected in the Amazon are biological (such as pollen or spores) and tend to increase in size due to the deposition of volatile organic compounds. As these particles increase in size, they become more efficient at collecting liquid water or ice, thereby serving as condensation nuclei. The type of particle also influences condensation and cloud formation. For example, a particle that is typical of tropical forests is isoprene (heat stress) and its atmospheric concentration in tropical forests is higher than in herbaceous meadows. Isoprene has a great capacity to increase cloud cover and condensation nuclei and, therefore, rain during times of greatest heat stress. In addition, the wind lifts certain particles from the ground that facilitate the formation of ice at lower temperatures than usual.

As a consequence of the above, deforestation of forests can disrupt rainfall patterns. Due to the great importance of the biotic pump, deforestation can make an area dry and arid.